The field of the present invention is that of microbiological analysis. More particularly, the present invention relates to a device for preparing a biological sample with a view to a microbiological diagnosis.
Vacuum filtration is conventionally used in laboratories. It is used in two ways:                either to clarify a sample. The filtrate is then kept and the filter discarded,        or to collect and concentrate particles or microorganisms of interest present in a liquid sample. In this second case, the filtrate is discarded, and the concentrate (also called retentate) and/or the filter are kept.        
A known device from the prior art is the Büchner funnel. Such a device is represented in FIG. 1. It comprises a funnel in which a perforated plate is transversely positioned. The funnel is fitted in a leaktight manner into the neck of a vacuum flask by means of a bung. The vacuum flask is connected to a suction source. A filter paper is placed on the perforated plate, said filter paper having been moistened so that particles present in a liquid poured onto the upper part of the paper can be retained during the suction. The liquid filtered, or filtrate, is collected in the flask.
Such a device has several drawbacks. Indeed, the filter can become rapidly blocked if the liquid contains too many particles or microorganisms, which can prevent filtration of the entire volume. Furthermore, the large filtration surface area, required in order to be able to filter a sufficient amount of sample, can require the use of a large volume of liquid for completely resuspending the microorganisms retained on the filter, which does not make it possible to have an optimal concentration ratio.
After having filtered the sample by means of this device, the collection of particles of interest or microorganisms retained on the filter paper is difficult and requires the manual use of a swab. It is not easy to reproducibly perform the recovery by swab automatically. Furthermore, this type of device has an unfavorable second release yield if the concentrate, in particular bacterial concentrate, must be provided in liquid form for the needs of the analysis protocol. Finally, during the steps of washing the filter or of taking up the microorganisms with a pipette, the liquid dispensed spreads over the entire surface of the filter, and it is therefore difficult to again suction it. As a result, the sequence of a filtration protocol using this device cannot be easily automated.
Another device of the prior art, described in patent application US 2007/0298451 A1, comprises a lower filtration block and an upper filtration block. The upper filtration block comprises a cylindrical wall having a first diameter and a cylindrical wall having a smaller diameter, connected by a transverse part. A perforated plate and also a filtration membrane are placed on the transverse part. The lower filtration block comprises a cylindrical wall having a first diameter and a cylindrical wall having a smaller diameter connected by a transverse part. A perforated plate and also a filtration membrane are placed on the transverse part. A cylindrical wall having an even smaller diameter is placed downstream of the perforated plate and connected to the lower filtration block by a transverse part, so as to connect a suction source to the device.
The upper filtration block is fitted onto the lower filtration block. The membrane of the upper filtration block has a porosity greater than the membrane of the lower filtration block. As a result, when a liquid sample is poured onto the membrane of the upper filtration block and the suction source is activated, said sample is filtered a first and then a second time.
Although this device can have a better filtration yield, in particular if progressive filtrations are carried out, the liquid dispensed also spreads over the entire surface of the two membranes, making collection difficult. Furthermore, blocking, even partial, of one or more membrane(s) can lead to a pressure drop during suctioning and slow down or even prevent filtration.